PSI - Issue 60

D. Sen et al. / Procedia Structural Integrity 60 (2024) 44–59 Deeprodyuti Sen/ Structural Integrity Procedia 00 (2024) 000 – 000

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‘ k t ’ are computed. The threshold peak stress is divided by ‘ k t ’ to obtain the allowable nominal stress that will not lead to DHC initiation from the volumetric flaw. A graph showing comparison of the allowable nominal stress, for various values of ‘ a ’ , ‘ ρ ’ and ‘ c ’ , with the applied stress in the pressure tube is then plotted to identify the safe and unsafe regions. A summary of salient design data that is used for evaluation of nominal stresses in pressure tubes used in 220 and 540 MWe Indian PHWRs, under the design conditions, is provided in Table 2. Apart from the hoop stress resulting from internal pressure, two different values of residual stresses, that is, 100 MPa and 150 MPa are considered in the regions near the rolled joints.

Table 2: Design data for Pressure tubes of Indian PHWR taken from Sharma et . al (2011) and Bhachawat(2011) Parameters 220MWe 540MWe Inside radius ‘ R i ’ 41.2 mm 51.7 mm

Design wall thickness ‘ w ’ Nominal wall thickness

3.07 mm 3.32 mm

4.1 mm 4.3 mm

Internal pressure for service loading ‘ p r ’ Hoop Stress Inlet Temperature Outlet temperature Residual stress adjacent to rolled joint

9.7 MPa 157.18 MPa

11.2 MPa 165.77 MPa

249 o C 293 o C 100 MPa,150 MPa

260 o C 304 o C 100 MPa, 150 MPa

Numerical calculations for permissible flaw size were carried out using the in-house ZIPTAS code. The range of input parameters used in the iterative procedure is listed in Table 3 for 220MWe and 540MWe Indian PHWRs. Table 3: Range of input parameters used in the ZIPTAS code for evaluation of maximum permissible flaw size in a pressure tube used in 220 MWe and 540 MWe Indian PHWR Sl.No. Geometric Parameters 220 MWe 540 MWe 1 Flaw Depth, ‘ a ’ 0.1 mm to 0.75 mm in steps of 0.025 mm 0.1 mm to 1.025 mm in steps of 0.025 mm 2 Flaw root radius, ‘ ρ ’ 0.1 mm to 0.75 mm in steps of 0.025 mm 0.1 mm to 0.5 mm in steps of 0.025 mm 3 Half axial length, ‘ c ’ 0.16 mm to 30 mm 0.16 mm to 30 mm The upper limit for flaw depth ‘ a ’ is restricted to 0.75 mm and 1.025 mm as the procedure is valid for a maximum flaw depth of 0.25 times the wall thickness, which is around 3 mm for 220 MWe and 4.1 mm for 540 MWe Indian PHWRs respectively . For a given value of ‘ a ’, the flaw tip root radius ‘ ρ ’ is varied from a minimum value of 0.1 mm to the maximum permitted valu e of ‘ a ’. The minimum value of ‘ c ’ should be at least 1.5 times higher than the minimum value of ‘ a ’. We now provide the numerical values of the maximum permissible flaw size that will not lead to initiation of DHC under the design loads. Nominal operating stress resulting solely from design internal pressure is shown as dashed line. The combination of radial flaw depth ‘ a ’, flaw tip root radius ‘ ρ ’ and axial crack length ‘ 2c ’ for which the allowable stress is higher than the operating stress will not lead to DHC initiation and, therefore, can be considered as safe.